Polyvinyl alcohol (hereinafter referred to as "PVA") is known as a representative vinyl alcohol polymer, which has vinyl alcohol units in the molecule thereof. PVA is excellent in film formability, transparency and strength and hence has been widely used as quality-improving agent for paper, e.g. coating agent and additive for paper; adhesive for paper, wood, inorganic materials and the like; sizing agent for warp; films, sheets and the like. Conventional PVAs are classified into "fully hydrolyzed PVA" having a degree of hydrolysis of about 98 mole % and "partially hydrolyzed PVA" having a degree of hydrolysis of about 88 mole %.
These conventional PVAs have been used in the form of aqueous solution because they are insufficient in thermal stability. Hence, fully hydrolyzed PVA with its melting point being very close to its thermal decomposition temperature cannot be hot-melt molded. On the other hand, partially hydrolyzed PVA, which has a lower melting point as compared with fully hydrolyzed PVA, has a poor thermal stability and hence produces acetic acid odor during hot-melt molding.
There have been proposed processes for hot-melt molding of PVA, which comprise decreasing the melt viscosity of PVA by blending therewith a plasticizer or another polymer. However, in the case of the method of addition of a plasticizer in the molded articles, the content of the plasticizer decreases with time when the molded articles are used for a long period of time. As a result, the molded articles become insufficient in flexibility thereby causing splits or cracks under low temperature and low humidity conditions as in winter. On the other hand, the method of blending PVA with another polymer gives molded articles with very poor mechanical properties or transparency, since compatibility of each polymer is insufficient.
Alternatively, modification of PVA has been proposed to lower the melting point of PVA.
However, PVA with (.omega.-hydroxyalkyl vinyl ether units and an alkyl vinyl ether units (Japanese Patent Publication No. 10885/1992) and PVA with polyoxyethylene monoallyl ether units (Japanese Patent Publication No. 49683/1993) have a problem of poor thermal stability because of their ether bonds. PVA with allyl alcohol units (Japanese Patent Application Laid-open No. 229135/1987) has a problem that its thermal stability is still insufficient for practical purposes although its thermal stability is improved to some extent and has a problem of the allyl alcohol remaining in the PVA with regard to safety. PVA with .alpha.-olefin units (Japanese Patent Application Laid-open No. 289581/1988) has an extraordinary high melt viscosity and is water-insoluble, due to association of hydrophobic groups.